1. Field of the Invention
The present invention relates generally to manufacture of heads for data storage devices and more specifically to a read head for use with a perpendicular write head in a magnetic head for a hard disk drive.
2. Description of the Prior Art
Data has been conventionally stored in a thin media layer adjacent to the surface of a hard drive disk in a longitudinal mode, i.e., with the magnetic field of bits of stored information oriented generally along the direction of a circular data track, either in the same or opposite direction as that with which the disk moves relative to the transducer.
More recently, perpendicular magnetic recording systems have been developed for use in computer hard disk drives. A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil around the write pole. In this type of disk drive, the magnetic field of bits of stored information are oriented normal to the plane of the thin film of media, and thus perpendicular to the direction of a circular data track, hence the name.
Media used for perpendicular recording typically include a hard magnetic recording layer and a soft magnetic underlayer which provides a flux path from the trailing write pole to the leading opposing pole of the writer. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the opposing pole, completing a flux loop.
Perpendicular recording head designs have the potential to support much higher linear densities than conventional longitudinal designs. Magnetization transitions on the bilayer recording disk are recorded by a trailing edge of the trailing pole and reproduce the shape of the trailing pole projection on the media plane, thus the size and shape of the pole tip is of crucial importance in determining the density of data that can be stored. Perpendicular magnetic recording is expected to supersede longitudinal magnetic recording due to the ultra-high density magnetic recording that it enables. This high density of magnetic flux can have an effect on the stability of the read head of the disk drive.
There are two configurations of read head in common use in the industry today. These are called Current Perpendicular to the Plane (CPP), and Current In the Plane (CIP). In the CPP configuration, the layers above and below the read sensor are made of conducting material which act as electrodes supplying current to the read sensor which lies between them. For purposes of this discussion, the read head will be assumed to be a CIP configuration.
A read head sensor 60 is generally formed of a wafer stack 67, such as is generally shown in a front plan view as seen from the Air Bearing Surface (ABS) in FIG. 5 (prior art). Shield layer S1 61 is formed on a substrate 70, followed by a gap layer, designated Gl 62. The wafer stack 67 is formed on the Gl 62 layer and includes an antiferromagnetic layer 74, a pinned layer 76, a spacer layer 78, a free magnetic layer 80, and a cap layer 82. A G2 layer is formed on the cap layer, but is not shown here.
Adjacent to the wafer stack 67, the read head 60 typically includes two hard bias layers 58 on either side of the wafer stack 67. Each hard bias layer 58 is formed on a seedlayer 56, and in a CIP configuration, a lead layer 59 is fabricated on the hard bias layer 58.
Generally, the magnetic moment of the free layer is free to rotate laterally within the layer with respect to the ABS from a quiescent or zero bias point position in response to magnetic field signals from data bits located on the rotating magnetic disk. The sensitivity of the sensor is quantified as the magnetoresistive coefficient dR/R where dR is the change in resistance of the sensor from minimum resistance to maximum resistance and R is the resistance of the sensor at minimum resistance.
The free layer material is very soft material, magnetically speaking, with very low coercivity, which is a measure of the minimum field strength necessary to make changes in the orientation of the magnetic domains. The free layer material necessarily must have this quality, as it is this layer's changes in magnetic alignment in response to the magnetic flux in the data bits in the data disk that leads to changes in resistance, which is how the data is read. This free layer material may therefore very easily fall into a multidomain state, where alignment of magnetic domains is not uniform. It is desirable however that the magnetization of the free layer be biased to be uniform throughout the entire layer. When the current is conducted through the sensor without magnetic field signals from the rotating magnetic disk, this is known as the quiescent position of the magnetic moment of the free layer. The preferred biased direction is parallel to the ABS.
If the free layer material is allowed to assume a multidomain state, the read signal will be degraded. In order to maintain the biased alignment during the quiescent state to produce better signal quality, there must be some external field applied to the free layer. The application of this external field is known as “stabilization”.
Permanent magnet films, referred to as “hard bias layers”, have been used to maintain the single-domain state in the free layer. The magnetic biasing field provided by the bias film must be sufficiently high to achieve stabilization and must itself be sufficiently stable to maintain its magnetization in the presence of high magnetic flux from the media which is being read.
A plot of the magnitude of the magnetization or flux density as a function of applied magnetic field is shown in a hysteresis loop 300, as illustrated in FIG. 8. A magnetic field H 302 has been applied to a sample of magnetic material, and the component of magnetization in the direction of H has been plotted as M 304. The maximum value of magnetization reached is called the saturation magnetization Ms 306, where the application of further H produces no increase in M. When the field H is reduced to zero, the value of M at this point is called the remanent magnetization Mr 308. The value of H for zero magnetization is called the coercive force or coercivity Hc 310. Squareness is a measure of the ratio of the remanent magnetization Mr to the saturation magnetization Ms.
Ferromagnetic materials are broadly divided into soft magnetic materials and hard magnetic materials. Hard magnetic materials exhibit low initial permeability and high coercive force and soft magnetic materials exhibit high initial permeability and low coercive force. FIG. 9 shows a hysteresis loop 320 for an exemplary soft magnetic material and FIG. 10 shows a hysteresis loop 340 for an exemplary hard magnetic material. It can be seen that the coercivity value Hc 350 is larger for the hard magnetic material than the coercivity value Hc 330 for the soft magnetic material. Squareness, being a ratio of the remanent magnetization to the saturation magnetization, is a dimensionless figure of merit that gauges the quality of the hard magnetic film.
The coercivity value Hc thus can be considered as a measure of how well a magnetic material maintains its magnetization in the presence of magnetic flux fields. As referred to above, the hard bias layer is required to have high coercivity, so that it is stable in the presence of magnetic flux, and can therefore stabilize the free layer by its biasing effect. This has become particularly critical for perpendicular recording apparatus where the read head is exposed to high levels of magnetic flux from the recorded media. It has been found that the choice of the seedlayer upon which the permanent magnetic film of the hard bias layer is to be formed has a great effect on the coercivity of the finished hard bias layer.
Tungsten (W) has been used as a very thin layer between Cr and the hard bias material to boost the coercivity. For example, the coercivity of a structure of a seed layer of Cr with a layer of CoPt hard bias material has been found to be around 1600 Oe. However when a 5Å layer of tungsten is fabricated upon the Cr layer followed by the CoPt layer, the coercivity is increased to more than 2200 Oe. Typical squareness values of 0.83 are obtained by using this structure containing tungsten. However, it is desired that higher coercivity levels be achieved to provide increased stability of the read head for perpendicular recording purposes.
Thus there is a need for a magnetic read sensor having a hard bias layer with improved coercivity and squareness in order to improve read head stability and overall disk drive performance.